This invention relates generally to systems and methods which utilize signals obtained from a living body and more particularly, to systems and methods of analyzing a human body and controlling prosthetic devices using acoustic signals obtained from skeletal muscles alone and in combination with myoelectric signals.
Considerable effort has been expended in prior years toward the extraction and analysis of electrical signals generated within living bodies. Of particular interest here are myoelectric signals which are understood to be representive of electrical excitation in skeletal muscles. It is now understood that myoelectric signals originate with the depolarization of the membranes of cells of individual muscle fibers during contraction. Such depolarization causes the generation of electrical potentials and currents which are detectable at remote locations, such as the surface of the skin. Thus, noninvasive techniques can be used to obtain the myoelectric signals, and therefore, such signals have been useful in controlling elementary prosthetic devices.
Ordinarily, myoelectric signals are obtained by placing an electrode, which may be made of a conductive, noncorrosive metal, such as silver or gold, on the surface of the skin of a living being. It is now well known that the placement of the electrode on the surface of the skin is a critical maneuver since precise placement of the electrode on the skin is required if a satisfactory signal detection is to be achieved. Generally, any slippage of the electrode from its initial location will degrade signal transmission.
In addition to the foregoing, myoelectric signal detection is adversely affected by variations in skin condition. For example, the impedance of the electrical communication between the electrode and the skin is altered substantially by the presence of perspiration. Thus, the electrical characteristics of the coupling to the skin of the electrode vary with skin condition. This is a substantial disadvantage of systems which rely upon myoelectric signals, in view of the very small amplitude of such signals.
In addition to requiring direct contact with the skin, myoelectric systems are subject to disruption by the presence of stray electrical fields. Accordingly, substantial electrical shielding is required, thereby increasing the cost and complexity of such systems.
It is a further problem with myoelectric signals that they do not contain within them complete information which characterizes muscular activity. In other words, the myoelectric signals are not representative of muscle activity, particularly after the onset of fatigue. During fatigue, excitation-contraction coupling is substantially reduced, and may in fact be near zero. Under such conditions, electrical activity of a muscle, as evidenced by the characteristics of a myoelectric signal, may appear to be normal, but little or no muscle contraction may be present. Thus, there is a need for a system which can assist in the determining of the onset of fatigue.
It has been known at least since the early nineteenth century that a rumbling-type of noise is produced when muscles are contracted. This noise-making capacity of skeletal muscles was publicized in the publication Philosophical Transactions of the Royal Society, pages 1-5 (1810). In this early lecture, Doctor William Hyde Wollaston describes a noise produced by contracting musculature having a frequency generally between 20 and 30 cycles per second, and amplitude which varies with the degree of force exerted by the muscle.
Much more recently, Doctors Oster and Jaffe reported in the Biophysical Journal, Vol 30, April 1980, pp. 119-128, in a paper entitled "Low Frequency Sounds from Sustained Contraction of Human Skeletal Muscle", that the sound produced by a muscle grows louder with the increased loading. The sound is quite loud at the commencement of the loading, but rather quickly settles to a steady volume. Such a sound is further reported as arising in the muscles themselves, and is not of vascular origin.
The acoustic signals generated by muscles, in the form of a relatively low frequency rumbling noise, can be detected by a transducer, such as a microphone, which need not be placed in direct communication with the surface of the skin. In fact, the skin can be covered by a sock. Such a covering may be particularly useful in situations where skin conditions, such as those requiring dressing or ointment, render direct communication between the microphone transducer and the skin undesirable. However, the amplitude of the acoustic signal received by the transducer decreases substantially absent direct communication between the transducer and the skin, and of course, with distance from the skin.
It is, therefore, an object of this invention to provide a system for producing signals responsive to muscle activity.
In is another object of this invention to provide a system for detecting muscle fatique.
It is a further object of this invention to provide an arrangement for monitoring signals pertaining to muscular activity in a living being without being affected by skin condition, or changes in skin condition over time, such as impedance changes which occur as a result of perspiration.
It is also an object of this invention to provide a system for producing a signal responsive to muscle activity without requiring contact with the skin.
It is yet another object of this invention to provide a system for evaluating postural problems.
It is yet a further object of this invention to provide a system for evaluating and diagnosing dynamic muscular problems, such as those which result in improper gait.
It is also another object of this invention to provide a noncontact system for facilitating biofeedback training.
It is still another object of this invention to provide a muscle activity detection system for analyzing fatique time for the muscles of patients, particularly patients on respirators.
It is still a further object of this invention to provide a system for providing signals for controlling prosthetic devices.
It is yet still another object of this invention to provide an arrangement for detecting muscle activity utilizing a transducer which is not as location sensitive as myoelectric electrodes.
It is an additional object of this invention to provide a system for producing signals responsive to muscle activity, the system being generally unaffected by nearby electrical fields.
Additionally, it is an object of this invention to provide a muscle monitoring system which does not require substantial shielding against electrical fields.
It is a further additional object of this invention to provide a system for producing a signal coresponding to muscle activity, the system having high signal-to-noise ratio.
It is yet an additional object of this invention to provide a muscle activity monitoring and detection system which is simple, inexpensive, and which can utilize circuitry of the type used to analyse myoelectric signals.